Ion-Substituted Glasses Through Different Routes of Synthesis for Glass-Ionomer Cements

Objectives: To determine if ion-substituted glass particles (partial substitution of strontium with magnesium and zinc ions) obtained though different routes of synthesis affect the physical properties of glass-ionomer cement (GIC)
Methods: Strontium-based alumino-fluorosilicate glasses of different compositions were obtained using two routes. A known glass, LG99Sr, (based on glass precursors of molar ratios, 4.5SiO₂-3Al₂O₃-1.5P₂O₅-3SrF₂-2SrO) and an experimental glass, LG99Sr-Mg-Zn, of novel composition 4.5SiO₂-3Al₂O₃-1.5P₂O₅-3SrF₂-0.5SrO-1MgO-0.5ZnO were synthesized using conventional melt-quench method. A new hydrolytic sol-gel method was an alternative route used to obtain glasses of similar compositions as the melt-processed glass. Sodium metasilicate served as the starting silica-precursor instead of traditional tetraethyl orthosilicate for the sol-gel process. Silica extracted was dissolved in citric acid to form a complex used to encapsulate other glass precursors. Precursors used were nitrates of corresponding oxides used for the melt process at same ratios. The silica-citric acid-precursor moiety was burnt-out and calcined to obtain the sol-gel glass. Each batch of melt and sol-gel glasses was milled to obtain ≤45μm particles. Glass characterisation was performed using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy. Compressive strength (CS), flexural strength (FS), and microhardness (VHN) of GIC groups were determined in accordance with ISO-specifications after storage of the specimens in distilled water for 24h at 37°C. Results were analysed using two-way ANOVA and Tukey's post-hoc tests at significance level of 0.05.
Results: At 24h setting, the CS, FS and VHN values of melt-processed glasses, LG99Sr and LG99Sr-Mg-Zn were significantly higher (p<0.05) than the respective sol-gel glasses of similar compositions. However, there was no significant difference (p>0.05) when comparing the mechanical properties of LG99Sr and ion-substituted LG99Sr-Mg-Zn prepared using the same synthesis route.
Conclusions: Melt-processed glasses had significantly higher mechanical properties than sol-gel glasses of similar composition. However, the compressive strength of sol-gel glasses still met the ISO-minimum standards for restorative GICs.